Mechanical Activation of Graphite for Na‐Ion Battery Anodes: Unexpected Reversible Reaction on Solid Electrolyte Interphase via X‐Ray Analysis

Abstract Although sodium‐ion batteries (SIBs) offer promising low‐cost alternatives to lithium‐ion batteries (LIBs), several challenges need to be overcome for their widespread adoption. A primary concern is the optimization of carbon anodes. Graphite, vital to the commercial viability of LIBs, has a limited capacity for sodium ions. Numerous alternatives to graphite are explored, particularly focusing on disordered carbons, including hard carbon. However, compared with graphite, most of these materials underperform in LIBs. Furthermore, the reaction mechanism between carbon and sodium ions remains ambiguous owing to the structural diversity of disordered carbon. A straightforward mechanical approach is introduced to enhance the sodium ion storage capacity of graphite, supported by comprehensive analytical techniques. Mechanically activated graphite delivers a notable reversible capacity of 290.5 mAh·g−1 at a current density of 10 mA·g−1. Moreover, it maintains a capacity of 157.7 mAh·g−1 even at a current density of 1 A·g−1, benefiting from the defect‐rich structure achieved by mechanical activation. Soft X‐ray analysis revealed that this defect‐rich carbon employs a sodium‐ion storage mechanism distinct from that of hard carbon. This leads to an unexpected reversible reaction on the solid electrolyte surface. These insights pave the way for innovative design approaches for carbon electrodes in SIB anodes.

Pristine graphite exhibited a distinct C-C bond peak (~284.8eV) with a minor presence of oxygen functional groups. [1]However, after ball milling, the intensity of the C-C bonds decreased, whereas the number of oxygen functional groups increased.This is consistent with the NEXAFS results.The atomic oxygen concentrations in the samples, calculated from the XPS data, increased from 3.08 % in pristine graphite to 12.15 % in 12 h-MG graphite.As previously highlighted, oxygen originated from the dry air inside the zirconia crucible.As milling proceeded, ZrO2 partially eroded from the surface of the crucible and balls, leading to a corresponding increase in its concentration.However, after 12 h of milling, the oxygen concentration plateaued due to the exhaustion of available oxygen in the air.Before the electrochemical reaction, the bare 4 h-MG was primarily composed of carbon at the surface and in the bulk region (Figure S6A).After sodiation, sodium concentrations at the surface and bulk regions in the inner and outer parts (post 50 min of etching) increased to 51 and 21 %, respectively.This indicates the occurrence of a sodiation reaction throughout the sample (Figure S6B).In the desodiated state, Na concentrations were 34.10 % (surface) and 18.92 % (bulk).This was attributed to the irreversible trapping of sodium ions within the structure (Figure S6C).The varying Na concentrations between the surface and bulk region during sodiation/desodiation suggest a surface-dominant reaction in the 4 h-MG.Furthermore, according to the atomic concentrations of oxygen and fluorine in the surface and bulk region post sodiation/desodiation, oxygen-and fluorine-containing compounds formed following 4 h of MG sodiation.The change in the atomic ratios of fluorine and sodium indicated that the initial low efficiency of the 4 h-MG originated from forming an SEI at the surface and irreversible reactions in the bulk region.Contrastingly, the 12 h-MG was predominantly carbon-based (Figure S6D) before sodiation, akin to the 4 h-MG.
However, the sodium distribution in the sodiated 12 h-MG was 60 % (surface) and 21 % (bulk) (Figure S6E), exceeding that in the 4 h-MG.After desodiation (Figure S6F), the surface sodium concentration (31 %) of the 12 h-MG was comparable to that of the 4 h-MG.However, the bulk sodium concentration of the 12 h-MG (15 %) was lower than that of the 4 h-MG.This result indicates that the 12 h-MG trapped less irreversible sodium than the 4 h-MG.The variance in sodium storage behavior in the bulk region between the 12 h-MG and 4 h-MG originates from their diverse interlayer spacing and structural properties.Furthermore, despite the extensive surface area of the 12 h-MG, its surface sodium concentration was higher than that of the 4 h-MG, suggesting that sodium storage at the surface was not proportional to the surface area.Moreover, the oxygen concentration in the bulk region of the 12 h-MG significantly increased during sodiation (1.74 %) and desodiation (4.08 %).Conversely, the 4 h-MG displayed consistent oxygen concentrations in its bulk region during the sodiation (1.35 %) and desodiation (2.00 %).The depth profile detailing the atomic concentration of pristine graphite was examined before and after sodiation under identical conditions (Figure S7).There was no significant increase in Na concentration either on the surface or within the bulk region of graphite, even after sodiation, which is consistent with previous studies.The ex situ XPS profile for 4 h-MG (Figure S9) resembled that for 12 h-MG.However, there were significant differences in the C1s, O1s, and Na1s spectra of the sodiated and desodiated states.Therefore, the sodium ion behavior at the surface and bulk regions of the 12 h-MG was more pronounced than that in the 4 h-MG.

Figure
Figure S4 A) N2 isotherm curves and B) pore size distributions of the samples (inset; highly-magnified pore size distributions)

Figure S5 .
Figure S5.A) Schematic diagram of coin cell.CV curves and b value calculation of (B, C) 4 h-MG and D, E) 12 h-MG.

Figure S6 .
Figure S6.Atomic concentration measured by XPS sputter depth profile before and after sodiation and desodiation states of (A-C) 4 h-MG and (D-F) 12 h-MG.

Figure
Figure S6 shows the atomic concentrations of carbon, oxygen, fluorine, and sodium in the 4 h MG, plotted against sputtering time across varying states of charge (SoC).

Figure S7 .
Figure S7.Atomic concentration measured by XPS sputter depth profile A) before and B) after sodiation of pristine graphite.

Figure S9 .
Figure S9.XPS depth profile of 4 h-MG with Ar + ion etch from surface to 40 mins etched A) C1s B) O1s C) F1s and D) Na1s

Figure S11 .
Figure S11.Ex-situ TEM images of fully sodiated 12 h-MG (A-B) HR-TEM images and selected area diffraction (SEAD) patterns.TEM-EDS mapping of C) carbon, D) oxygen, E) fluorine and F) sodium